Is it necessary to divorce a wife if the husband doubts her love for him, after he found pictures and confessions indicating previous relationships before marriage?

The default is to assume the best of a Muslim, especially between spouses. It is obligatory to deliberate and not rush into accusing the wife. The best approach is to be frank with her while showing trust in her. If it is certainly proven that her heart is attached to another young man, the divine remedy for a wife's nushooz (disobedience/rebellion) is summarized as: admonition, then abandonment in bed, then non-brutal beating. If she does not desist, then two arbitrators should be sent from their families, as Allah Almighty says: "As for those [wives] from whom you fear nushooz, admonish them, and abandon them in bed, and strike them. But if they obey you, seek no means against them. Indeed, Allah is ever Exalted and Grand. And if you fear a breach between them, appoint an arbitrator from his people and an arbitrator from her people. If they both desire reconciliation, Allah will cause reconciliation between them. Indeed, Allah is ever Knowing and Acquainted [with all things]." (Quran 4:34-35). The wife must dispose of pictures of non-mahram men, as it is impermissible to possess them and because they can be a means for her heart to become attached to them, which is a betrayal of the husband. The husband must destroy them as part of his responsibility.